As far as the original question, I think it depends on the details of the dislike. I tend to have a certain amount of baseline dislike for all therapists at first, mostly based on finding the whole enterprise of trying a new person in that role to be extremely difficult and frustrating. That kind of dislike fades for me over time, and hasn't prevented therapy from being effective.

On the other hand, there have been several therapists where my dislike clearly strayed outside that baseline, whether because the therapist was stupid, condescending, bigoted, a bad personality match, or several of the above. And that type of dislike was a clear contraindication to therapy being effective.

Finally, I've also on occasion experienced a more transient dislike of a therapist who I'd otherwise been working well with for an extended period of time. That was also not an obstacle to effective therapy when the therapist handled it appropriately.